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Enzyme activities of the isoleucine-valine biosynthetic pathway in streptomycin mutants of Escherichia coli Lau, D.C.C.


The compound α-acetolactate has been prepared by the chemical method of Krampltz (1948) and by an enzymatic procedure. The products obtained by each method were characterized chromatographically and spectrophotometrically by conversion to their 2,4—dinitrophenylhydrazone derivatives as well as by conversion to acetoin. The Identity of the hydrazone of enzymatically prepared α-acetolactate with hydrazones of the chemical product was established by comparison of R[subscript: F] values, the identical absorption maxima, and by the infrared spectra of these compounds. The α-acetolactate so obtained was used as substrate in a comparison of the activity in streptomycin mutants of the reductoisomerase enzyme which catalyzes the rearrangement and reduction of α-acetolactate to α,β-dihydroxyisovaleric acid. This reaction is the second step in valine biosynthesis. Streptomycin-dependent mutants of Escherichia coli previously has been shown to be derepressed in acetohydroxy acid synthetase, the enzyme which initiates valine biosynthesis. In addition, it had been reported previously that in streptomycin-dependent E. coli K-12, threonine dehydratase, the enzyme which initiates biosynthesis of isoleucine, also is derepressed. In contrast, reductoisomerase, which is common to both the valine and isoleucine pathway, has been found in this work to be normal (i.e., not derepressed) in streptomycin mutants. An additional enzyme of the common pathway, transaminase B, was found to be about 2 to 3 times higher in streptomycin-dependent mutants than in sensitive or resistant strains. The structural genes for both transaminase B and threonine dehydratase of E. coli K-12 have been shown by genetic studies (Ramakrishnan and Adelberg, 1965b) to be coordinately regulated (i.e., on the same operon). The observations made in this study with streptomycin-dependent E. coli K-12 support the observations of these workers. That is, transaminase B of streptomycin-dependent E. coli K-12 is derepressed coordinately with threonine dehydratase. However, the degree of derepression of transaminase B (about 2 to 3 fold) was much less than that of threonine dehydratase (about 9 fold, according to Desai and Polglase, 1966). It may be concluded from these studies that the type of derepression of certain enzymes which has been observed in streptomycin-dependent E. coli has contrasting features to the type of derepression which would be expected on the basis of the Jacob and Monod model (1961) from a nonfunctional regulatory gene or product thereof.

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